This invention relates generally to the field illustrated by "plugging" U.S. Pat. Nos. 4,262,187 and 4,513,786; "tube removal" U.S. Pat. Nos. 3,492,453 and 3,986,245; and "plug removal" U.S. Pat. 4,679,315, all assigned to the same assignee as the instant invention.
The invention is a tool for electron discharge machining (E.D.M.) to remove an installed tube plug in the end of a steam generator tube associated with a pressurized water type of nuclear reactor (PWR). At the lower end of a typical steam generator of this type, a tube sheet having an array of tube-end receiving bores separates the primary side of the heat exchanger, which includes the lower primary chamber and tube interiors of all the U-shaped tubes beginning and ending in the tube sheet (the tube bundle), from the secondary side which contains the water and steam heated in the heat exchanger for use by electric power generating turbines in communication with the steam generator.
Plugging the occasional leaking tube or the more common degraded tube is now frequent in PWR plant operation. A recent development is the emphasis on tube plugs that can be removed easily, but in earlier repair operations, permanently welded or explosively expanded plugs were commonly used. These plugs, and even some plugs that are designed to be easily removed, often must be drilled out of the tube, either for inspection purposes, further repair purposes, or when it is necessary to repair and return a previously plugged tube to heat exchange service.
The drilling operation, when used, provides problems of time which result in high man-rem exposure rates for the workmen. This is due to the requirement of multiple entries of the workmen into the primary side chamber, drill breakage from drilling the radiation exposed hardened material, and drill lead-off into the tube sheet and the resulting need for extensive repair work.
Advantages which are available by use of the remote electron discharge machining tool of the invention are:
1 The E.D.M. process uses very little cutting pressure, thus the tooling exerts almost no forces against the plug or tube sheet to cause lead-off or breaking;
2 The E.D.M. process is a single pass process, with a minimum amount of entries into the steam generator to accomplish the task of tube plug removal;
3 The E.D.M. process removes no metal from the inside diameter of the tube, leaving a tube which could possibly be returned to service.
The process of electron discharge machining is generally known and includes the removal of material by spark erosion. The sparks are created by an electrical discharge across a gap between an electrode and a grounded conductive workpiece. The gap is typically filled with a dielectric fluid which contains constituents such as flourides, chlorides, sulfides and other materials which would be harmful to the Inconel tubing of a PWR steam generator.